FIG. 1 shows a schematic representation of the essential features of a typical prior-art fiber optic transceiver. The main circuit 1 contains at a minimum transmit and receiver circuit paths and power 19 and ground connections 18. The receiver circuit, which takes relatively small signals from an optical detector and amplifies them to create a uniform amplitude digital electronic output, typically consists of a Receiver Optical Subassembly (ROSA) 2 which contains a mechanical fiber receptacle as well as a photodiode and pre-amplifier (preamp) circuit. The ROSA is in turn connected to a post-amplifier (postamp) integrated circuit 4, the function of which is to generate a fixed output swing digital signal which is connected to outside circuitry via the RX+ and RX−pins 17. The postamp circuit also often provides a digital output signal known as Signal-Detect or Loss-of-Signal indicating the presence or absence of suitably strong optical input. The Signal-Detect output is provided as an output on pin 20.
The transmit circuit, which accepts high speed digital data and electrically drives an LED or laser diode to create equivalent optical pulses, typically consists of a Transmitter Optical Subassembly (TOSA) 3 and a laser driver integrated circuit 5. The TOSA contains a mechanical fiber receptacle as well as a laser diode or LED. The laser driver circuit will typically provide AC drive and DC bias current to the laser. The signal inputs for the AC driver are obtained from the TX+ and TX−pins 12. Typically, the laser driver circuitry will require individual factory setup of certain parameters such as the bias current (or output power) level and AC modulation drive to the laser. This is accomplished by adjusting variable resistors or placing factory selected resistors 7, 9 (i.e., having factory selected resistance values). Additionally, temperature compensation of the bias current and modulation is often required because the output power of laser diodes and LEDs can change significantly across a relatively small temperature range.
The prior art fiber optic transceiver of FIG. 1 uses thermistors (e.g., thermistors 6, 8) whose electrical resistance changes as a function of temperature to control the current supplied to the laser diodes. Under high-volume manufacturing conditions, however, the temperature compensation scheme using thermistors is inaccurate due to variations in thermistor characteristics and laser characteristics.
Accordingly, what is needed is a method of maintaining desirable optical power of the optical emitters over temperature variations. What is further needed is a temperature compensation mechanism that is not vulnerable to variations in thermistor characteristics and emitter characteristics.